An Adaptable Binary Function is a Binary Function with
nested typedefs that define its argument types and result type. [1][2]
These nested typedefs make it possible to use function object adaptors.
[1]
Note the implication of this: a function pointer T (*f)(X,Y) is
a Binary Function, but not an Adaptable Binary Function: the
expressions f::first_argument_type, f::second_argument_type, and
f::result_type are nonsensical.
[2]
When you define a class that is a model of Adaptable Binary Function,
you must provide these typedefs. The easiest way to do this is to
derive the class from the base class binary_function. This is an
empty class, with no member functions or member variables; the only
reason it exists is to make defining Adaptable Binary Functions
more convenient. Binary_function is very similar to the base classes used by
the iterator tag functions.
